Can't drag or rearrange topic

What would be the reason that I would be unable to move/drag/repositiion a topic on a map? I have a level 2 topic, with a dozen subtopics. I created a new second level topic, and am trying to drag some of the children of the first level two, onto it. I get a "not" icon (circle with a slash through it).

Why would I be able to drag and drop some of the children and not others?

I just exported the two main tasks to linked maps and found that in some cases the child content of 2nd or lower level topics simply would not transfer. I had to manually create the new structure and was able to manually copy over the leaf level nodes and go everything straightened out manually.